1. Field of the Invention
The present invention relates to a medium access control (MAC) protocol in a wireless ad hoc network system. Specifically, a communications system and method with a handshaking protocol is used for data exchange in a single or multi-channel, multihop wireless network including a local scheduling algorithm to resolve fairness and service differentiation problems in a fully distributed system.
2. Description of the Related Art
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed for use by the military. In this type of network, each mobile node is capable of operating as a base station or router for other mobile nodes, thereby eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks, such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, in U.S. patent application Ser. No. 09/815,157 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, filed on Mar. 22, 2001, and in U.S. patent application Ser. No. 09/815,164 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, filed on Mar. 22, 2001, the entire content of each being incorporated herein by reference.
Communication between nodes, however, is often subject to errors due to interference, multipath and fading effects and collisions. Avoidance of many such errors can be achieved using a control-signal handshake between transmitting and receiving nodes. A communication protocol, such as multiple access with collision avoidance (MACA), uses such a handshake technique between nodes consisting of a request-to-send (RTS) control packet sent from a source node to a destination node which in response, replies with a clear-to-send (CTS) control packet. However, a MACAW algorithm typically handles ARQ retransmissions for corrections of such errors by repeating the whole request-to-send/clear-to-send (RTS/CTS) channel access sequence. In addition, MACAW introduced data-sending (DS) messages to form RTS-CTS-DS-DATA-ACK message exchange and a new backoff algorithm. The IEEE 802.11 MAC is a variation of CSMA/CA protocol that implements both carrier sensing and virtual (RTS-CTS exchange) carrier sensing with acknowledgment messages to improve reliability.
Accordingly, a need exists for a medium access control protocol that can detect and schedule communications in a wireless ad hoc network system to avoid such errors described above, and eliminate the complications associated with corrections.
As discussed in U.S. Pat. No. 6,556,582 issued to Redi, the entire content of which is being incorporated herein by reference, a method to avoid multiple access collision in a MACA based wireless network is described. This collision avoidance scheme is proposed for a system with multiple transceivers, i.e. data channel and reservation channel transceivers. Since the transceivers work simultaneously, no reservation channel transmission is missed. NCTS messages are used to inform other nodes that the intended destination node is busy. Accordingly, a need exists to resolve the problems related to having one transceiver and one or multiple data channels, such as missing reservation data. The method described in the Redi Patent includes also using a priority field in RTS messages. However, processing of this information is based on an absolute comparison, i.e. the relative status of the transmissions in the neighborhood is not used and fairness is not studied.
As discussed in U.S. Pat. No. 6,118,788 issued to Kermani et al., the entire content of which is being incorporated herein by reference, a method to provide fairness in MACA based wireless networks is described. The system considered in the Kermani Patent has only one channel where transmission durations are not distributed in RTS-CTS messages but via an end-of-the burst control frame after the transmission. Accordingly a need exists for address and channel monitoring that includes multiple data channel systems where transmission durations are advertised via RTS-CTS messages. The main objective in the Kermani Patent is to solve hidden terminal problems, but not to also resolve the problems related to having one transceiver and multiple data channels, such as missing reservation data. The proposed system in the Kermani Patent is based on the fact that the neighbors will have the full knowledge of the number, or some other information, of possible and logical connections in the neighborhood by distributing this information. For example, it is suggested that the neighbors that hear an advertised window size will adjust theirs accordingly. However if this information is not correct, this may cause every node to decrease their backoff time and increase the collision. Accordingly a need exists for a method and system that uses feedback from transmissions and receiver assistant information correction to complete the information about the neighborhood and the link. Service differentiation is not studied in the Kermani Patent.
As discussed in U.S. Patent Application Publication No. 20020154653, issued to Benveniste, the entire content of which is being incorporated herein by reference, a method with backoff adaptation to traffic for CSMA networks is described. The system considered is one-hop system with one channel. The method aims to improve IEEE 802.11 type networks in terms of contention delay and service differentiation. The method is mainly based on the estimation of the traffic intensity from the number of failed transmission attempts. However, in multihopping systems, one important cause of channel access delay is that many streams may be aggregated at one relaying node (such as wireless routers that do not have central controlling capabilities as access points in one-hop systems) and the relaying node may be busy. Therefore, although channel and address monitoring decrease the collision probability (hence, decrease the number of failed transmission attempts), the offered traffic intensity may be high. This delay may cause short term unfairness and slow down TCP type traffic, hence change the actual traffic intensity. Accordingly a need exists for a node weight to alleviate this problem.
Furthermore, a need exists to differentiate reservation channel and data channel collisions for a system with multiple channels. Another point is that, unlike the method described in the Benveniste Publication, streams to different next hops may exist at relaying nodes and therefore a backlogged packet due to the next hop busy status or link failure may be scheduled later than a newer packet destined to another next hop. Incomplete neighborhood information is a major problem in multihop, one or multiple channel networks and accordingly, a need exists for a system and method of comparison of local measurements (via unicast messaging) to alleviate these problems.